From: Finn Thain <fthain@linux-m68k.org>
To: Peter Zijlstra <peterz@infradead.org>, Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Boqun Feng <boqun.feng@gmail.com>,
Jonathan Corbet <corbet@lwn.net>,
Mark Rutland <mark.rutland@arm.com>,
Arnd Bergmann <arnd@arndb.de>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
Geert Uytterhoeven <geert@linux-m68k.org>,
linux-m68k@vger.kernel.org, linux-doc@vger.kernel.org
Subject: [RFC v3 1/5] documentation: Discourage alignment assumptions
Date: Wed, 08 Oct 2025 09:19:20 +1100 [thread overview]
Message-ID: <76571a0e5ed7716701650ec80b7a0cd1cf07fde6.1759875560.git.fthain@linux-m68k.org> (raw)
In-Reply-To: <cover.1759875560.git.fthain@linux-m68k.org>
Discourage assumptions that simply don't hold for all Linux ABIs.
Exceptions to the natural alignment rule for scalar types include
long long on i386 and sh.
---
Documentation/core-api/unaligned-memory-access.rst | 7 -------
1 file changed, 7 deletions(-)
diff --git a/Documentation/core-api/unaligned-memory-access.rst b/Documentation/core-api/unaligned-memory-access.rst
index 5ceeb80eb539..1390ce2b7291 100644
--- a/Documentation/core-api/unaligned-memory-access.rst
+++ b/Documentation/core-api/unaligned-memory-access.rst
@@ -40,9 +40,6 @@ The rule mentioned above forms what we refer to as natural alignment:
When accessing N bytes of memory, the base memory address must be evenly
divisible by N, i.e. addr % N == 0.
-When writing code, assume the target architecture has natural alignment
-requirements.
-
In reality, only a few architectures require natural alignment on all sizes
of memory access. However, we must consider ALL supported architectures;
writing code that satisfies natural alignment requirements is the easiest way
@@ -103,10 +100,6 @@ Therefore, for standard structure types you can always rely on the compiler
to pad structures so that accesses to fields are suitably aligned (assuming
you do not cast the field to a type of different length).
-Similarly, you can also rely on the compiler to align variables and function
-parameters to a naturally aligned scheme, based on the size of the type of
-the variable.
-
At this point, it should be clear that accessing a single byte (u8 or char)
will never cause an unaligned access, because all memory addresses are evenly
divisible by one.
--
2.49.1
next prev parent reply other threads:[~2025-10-07 23:50 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-07 22:19 [RFC v3 0/5] Align atomic storage Finn Thain
2025-10-07 22:19 ` [RFC v3 4/5] atomic: Add alignment check to instrumented atomic operations Finn Thain
2025-10-07 22:19 ` [RFC v3 3/5] atomic: Specify alignment for atomic_t and atomic64_t Finn Thain
2025-10-07 22:19 ` [RFC v3 2/5] bpf: Explicitly align bpf_res_spin_lock Finn Thain
2025-10-09 2:10 ` Alexei Starovoitov
2025-10-09 2:56 ` Finn Thain
2025-10-09 7:02 ` Peter Zijlstra
2025-10-09 15:17 ` Alexei Starovoitov
2025-10-09 16:01 ` Arnd Bergmann
2025-10-09 16:12 ` Alexei Starovoitov
2025-10-07 22:19 ` Finn Thain [this message]
2025-10-14 10:23 ` [RFC v3 1/5] documentation: Discourage alignment assumptions David Laight
2025-10-15 7:40 ` Finn Thain
2025-10-15 13:53 ` David Laight
2025-10-16 6:53 ` Finn Thain
2025-10-16 13:30 ` Arnd Bergmann
2025-10-16 22:14 ` Finn Thain
2025-10-07 22:19 ` [RFC v3 5/5] atomic: Add option for weaker alignment check Finn Thain
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